The incorrect page numbers can be caused by an incorrect reply count. Try running this:

Admin CP -> Tools -> Rebuild Caches -> Rebuild Thread Information

That will correct the reply count which should fix the page numbers.

Your reply count is obviously wrong for some reason. I don't know why. There was obviously a failure somewhere. This does not happen as a matter of course. Regardless of the cause, the rebuild utility will fix it.

Like I said, I have been unable to reproduce this problem on my test forum. I tried several applications of the spam cleaner in situations where the total number of pages would change. Everything worked fine.

Try disabling any addons you have installed. Maybe one of them is causing this problem.